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Old 13th May 2011, 08:57 PM   #1
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Unhappy Need help choosing mains fuse value


I recently built the LM3886 kit from, and I blow the mains fuse every time I turn it on. I'm using a single power supply ('s snubberized PS, see diagram). I'm using a 300 VA torroid with dual 18V secondaries (Antek AN3218, see other file). I've tried 3 and 4 amp slow blo, and 3 amp medium blo fuses. Amp boards are not hooked up yet. If the rectifier & filter caps are not attached, I measure ~18VAC from both secondaries - no blown fuses - so I'm pretty confident I've got the transformer wired correctly.

However, when I have the rectifier and filter caps attached and press the power switch:
  1. The on-board LED lights up very briefly
  2. one of the mains fuses (there are 2, one for live and one for ground) blows - usually live.
  3. the on-board LED slowly dims.
  4. Nothing on the board appears fried, or feels hot.
The instruction manual suggests a mere 2amp mains fuse for a 22VA transformer, and up to 3 amps for larger transformers. I consistently blow 4 amp 250V fuses.

So my problem is that I don't know whether I'm using too low rated fuses, or if there's a short in my board. I'm afraid to simply try higher rated fuses in case it's the latter. My multimeter tells me I don't have a short. Current flows between the inputs for each pair of windings very briefly - I guess it's charging up the filtering caps (pair of 10,000uf 50V). The fact that the indicator LED dims slowly after the fuse blows also suggests the filter caps were charging up.

So . . . Do I try higher rated fuses? If they keep blowing, when do I stop? Is there another way to test/troubleshoot the board?

Any advice is greatly appreciated!
Attached Images
File Type: gif ps.gif (9.7 KB, 167 views)
Attached Files
File Type: pdf AN-3218.pdf (36.7 KB, 4 views)
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Old 13th May 2011, 09:25 PM   #2
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Search for "Inrush current limiting" and "using a series mains light bulb" when testing amplifiers.

The latter will instantly tell you if there is a fault or just to much initial current. Torroids are bad at this.

Last edited by cliffforrest; 13th May 2011 at 09:26 PM. Reason: rubbish keyboard skills ....
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Old 13th May 2011, 09:26 PM   #3
WSJ is offline WSJ  United States
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National has an app note for an inrush circuit to limit the high current that occurs at power up.

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Old 13th May 2011, 09:32 PM   #4
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18VAC from an unloaded 18VAC secondary? That doesn't sound right. Set aside your confidence and make sure the transformer is wired correctly. Same for the rectifiers and filter caps.
The simple power calculation of 4 amps at 120VAC is 480 watts. Something is obviously wrong with your circuit. Nothing feels hot because power is dissipated by the fuse - in short order. Don't use a larger fuse, use a light bulb in series with the hot mains for current limiting. Do that only after you've thoroughly checked the circuit. Then checked it again.
It is error only, and not truth, that shrinks from enquiry. - Thomas Paine
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Old 13th May 2011, 09:33 PM   #5
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Id hook up a 150 watt light bulb in series. If it stays lit while the ps is on ( i assume the amps arent hooked up) there is something wrong with the ps.

With 300va and 20,000uf of caps, odds are the surge current is too high on turn on and you will need to install a soft start.

For a soft start id use this relay on the power supply outputs:
T7NS5D1-48 Tyco Electronics / P&B General Purpose / Industrial Relays

to bypass a CL-80 or CL-90 on the primary side
CL-80 GE Sensing / Thermometrics Thermistors

Your whole soft start would be a relay, a diode for the coil, and a ntc.
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Old 13th May 2011, 11:41 PM   #6
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Default Thank's everyone for the suggestions.

Thanks All,

I read a little article on the "light bulb" tester, but was hoping to avoid building one.Click the image to open in full size. (lazy). I also know that things such as inrush current limiters exist, but I was hoping to avoid building one of those too.Click the image to open in full size. I'm going to go up to a 5amp fuse, and if that blows, I'll build a light-bulb tester.

SofaSpud: I don't understand what you think is wrong. I measured 18VAC from the secondaries, which are supposed to be 18VAC. This is without the rectifier attached.

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Old 14th May 2011, 01:07 AM   #7
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Yeah, I'm very probably wrong to misread your measurements. But you can with reasonable care use a lamp that's handy and some gator clip test leads to do the light bulb test. No need to sidetrack into another project.
You can also try swapping the 10000uF caps with smaller values to get the time constant within the fuse's slo-blo time. This will at least tell you if you have a working circuit.
It is error only, and not truth, that shrinks from enquiry. - Thomas Paine
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Old 14th May 2011, 05:01 AM   #8
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Talking SUCCESS!

Who would have thought that the back surface of the MUR860 diodes are not insulated. I put tiny heat sinks on all the bridge diodes (MUR860). I figured it couldn't hurt. Well, the heat sinks and screws were touching each other. I removed them and . . .! I hooked everything up, and I'm listening to Yello's album Zebra right now. I put the 3 amp medium blo fuses in, as that's all I have left.

Obviously, this is my first amplifier. It's surprisingly quiet bolted to a wooden board with no ground or shielding. I think it's got more punchy bass and detail than the Sansui receiver I was using. Of course I'm biased. Wait until I put it in a proper chassis with shielding and grounding! I'll post photo's in the gallery forum when it's all done.

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Old 14th May 2011, 05:28 AM   #9
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Remember to use T fuses (slow blow)
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Old 15th May 2011, 11:09 AM   #10
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I'm using 3A 250Vac slow blow for my 225VA 20,000uF.
Since it still able to blows my 3A 250Vac fast blow while 2A 250Vac slow blow make the amp sounds quite soft.
3A x 250V = 750VA

750VA / your mains(125V?) = 5A or 6A?

Last edited by cent88; 15th May 2011 at 11:12 AM.
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